1. Field of the Invention
This invention relates to an apparatus for suppressing vibrations and quaky movements in travel of a mobile or automotive type crane like rough terrain cranes.
2. Prior Art
As illustrated in FIG. 5, mobile type cranes generally have a boom 3 pivotally supported on a vehicle body 2 which is supported on driven wheels 1, for pivoting movements about a horizontal shaft 5 through a boom uplifting cylinder 4. With a mobile crane of this sort, it is usually experienced that the vehicle body 2 is subjected to vibrations or quaky movements during travel due to undulations or irregularities on road surfaces or due to abrupt accelerations or decelerations of the vehicle body 2, putting the boom 3 and other attachments also in quaky movements to magnify the vibrations and quaky movements of the vehicle body 2 itself, giving ride discomfort to the operator on the vehicle.
For the purpose of damping such vibrations in travel, there has been known in the art an apparatus as disclosed in Japanese Laid-Open Patent Application 59-182195. As shown in FIG. 6, this prior art apparatus employs a damper mechanism 19 which is built in a boom uplifting cylinder 18. It includes a counter-balancing valve 12 provided in a conduit 13 in communication with an oil chamber 181 which holds the load of the cylinder 18, and an electromagnetic change-over valve 16 and a shuttle valve 15 which are provided between the conduit 13 and a conduit 17 which is connected to the other oil chamber 182 or a conduit 14 which is in communication with a direction control valve 11.
According to the prior art apparatus, if the direction control valve 11 is switched to a boom-up or boom-down position when the change-over valve 16 is in position A, the oil pressure is supplied to the oil chamber 181 or 182 of the cylinder 18 to expand or contract the cylinder 18 for moving up or down the boom. If the change-over valve 16 is switched to position B, the conduit 13 is communicated with the conduit 17 through the electromagnetic valve 16 and shuttle valve 15, forming a closed circuit through the oil chambers 181 and 182 and the oil chamber 191 of the damper mechanism 19 thereby to suppress quaky movements relative to a vehicle body 1 when the vehicle is in travel.
With this prior art apparatus, the change-over valve 16 is returned to position A by de-energization of its solenoid when the key of the vehicle is pulled out to turn off the power switch as in the case of the operator stopping the engine at a gas station for refilling fuel in the course of a vehicle driving operation with the change-over valve 16 held in position B for the vibration suppressing function. When re-starting the vehicle after refilling fuel, even if the power switch is turned on to re-start the engine, the change-over valve 16 remains in position A unless a damper switch is set in the travel position.
Namely, when re-starting the vehicle, the operator is required to switch the change-over valve 16 again into position B after re-starting the engine. If this procedure is neglected, the change-over valve 16 remains in position A during vehicle travel, failing to produce the damping effect and as a result making the ride uncomfortable.